1. Field of the Invention
This invention relates to modified polyolefin fibers preferred as a raw material for a hot-melt-adhesive polyolefin non-woven fabric and the non-woven fabric. More particularly, it relates to modified polyolefin fibers having a superior card-passing through property at the time of processing a non-woven fabric, and a low temperature adhesion and a broad width of processing temperature, and also having superior working characteristics on heating rolls, and a non-woven fabric.
2. Description of the Related Art
Hot-melt-adhesive type non-woven fabrics have been broadly used in view of advantages of provisions, economy, hygienics, etc., because the fabrics do not require any binder. Among the fabrics, polyolefin non-woven fabrics are superior in the properties and economy; hence they have been used in many fields such as those of surface materials for paper diaper, goods for menses, etc., medical or hygienic materials for operating gown, etc., civil engineering materials, agricultural materials, industrial materials, etc., and conjugate fibers of polyethylene/polypropylene and single fibers of polypropylene, etc. have been used as raw material fibers therefor.
The method for processing hot-melt-adhesive non-woven fabrics is roughly classified into an air-through process using hot air and heating roll process. Among these, the air-through process is applied to polyethylene/polypropylene conjugate fibers. Non-woven fabric obtained according to the air-through process is strong and soft, but the processing speed is slower than that by means of heating roll process and the processing is inferior in the productivity. Further, since polyethylene/polypropylene conjugate fibers are used, there is a drawback of a waxy feeling specific of polyethylene.
On the other hand, heating roll process has a high processing speed and a high productivity, and hot-melt-adhesion is carried out; hence even when polypropylene single fibers are used, it is possible to make a non-woven fabric from the fibers, and there is an advantage that there is no waxy feeling due to polyethylene.
However, it has been regarded as difficult to produce a polypropylene non-woven fabric provided with both of strength and a soft feeling according to heating roll process. The reason is as follows:
In order to produce the polypropylene non-woven fabric by means of heating roll, it is necessary to improve the hot-melt-adhesion between polypropylene fibers, and it is therefor necessary to produce the non-woven fabric at a high temperature at which the polypropylene fibers sufficiently soften at the time of hot-melt-adhesion bonding thereof.
However, when the non-woven fabric is produced at such a high temperature, the hot-melt-adhesion bonded points are deformed into a film form, and the polypropylene fibers other than the bonded points are also affected by heat, so that the feeling becomes inferior. Further, when processing is carried out at a low temperature, hot-melt-adhesion at the bonded points is insufficient, and strength endurable to practical uses cannot be obtained. Thus, the range of production conditions under which a propylene non-woven fabric being strong and having a soft feeling can be obtained, is very narrow, so that there has been raised a problem of production that strength lowers due to a small change in the processing temperature and the feeling becomes hard. Thus, it has been awaited to develop polypropylene fibers having a broad processing temperature for obtaining a soft and strong polypropylene non-woven fabric, and suitable to heat-rolling process.
Japanese patent application laid-open No. Sho 62-156310 proposes, as polypropylene fibers suitable to heat-rolling process, polypropylene fibers composed of ethylene-propylene random copolymer containing a definite quantity of ethylene and having a softening point of 132.degree. C. or lower. However, the fibers have drawbacks that they have hard feeling and the processing temperature range wherein it is possible to produce a non-woven fabric provided with both of strength suitable to practical uses and feeling, is very narrow.
Further, Japanese patent application laid-open No. Hei 2-112456 discloses a non-woven fabric composed of fibers of polypropylene having a specified isotactic pentad fraction. This non-woven fabric has a good feeling, but its strength is insufficient. Further, in general, the fibers composed of such a low stereoregular polypropylene have inferior carding characteristics; hence the fibers have drawbacks such that a serious problem occurs in the intrinsic production of non-woven fabric.
Further, Japanese patent application laid-open No. Hei 2-264012 proposes polypropylene fibers having 1,3,5-trimethyl-2,4,6-trisbenzene and dimyristyl-3,3'-thiodipropionic acid ester blended therein, but the feeling and tenacity thereof are both insufficient.
Problem to be Solved by the Invention
As described above, a number of attempts to provide a polypropylene non-woven fabric having superior strength and feeling according to heating roll processing have been carried out, but the properties of the produced non-woven fabrics are insufficient, and the processing temperature range at the time of production thereof is narrow; thus, satisfactory polypropylene fibers have not yet been developed.
The present inventors have made strenuous research in order to solve the above problems, and as a result, have found that a product obtained by adhering an oiling agent composed mainly of a dibasic acid ester and/or a fatty acid ester onto polyolefin fibers of low orientation having an index of birefringence of 0.054 or less exhibits the aimed high strength of non-woven fabric and softness even within a low processing temperature range, and have completed the present invention.
As apparent from the foregoing, the object of the present invention is to provide superior polyolefin fibers capable of easily producing a soft and high feeling polyolefin non-woven fabric, and having a broad processing temperature range and being suitable to heating roll processing.